The present invention relates to a liquid crystal composition comprising liquid crystal molecules and an alignment promoter. The invention also relates to an optically anisotropic element.
A liquid crystal display of transmission type comprises a liquid crystal cell and two polarizing elements placed on both sides of the cell. A display of reflection type comprises a reflection board, a liquid crystal cell and one polarizing element in this order.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates. The electrode layer has a function of applying a voltage to the rod-like liquid crystal molecules. Each of the substrates has an orientation layer, which has a function of aligning the rod-like liquid crystal molecules. Since the rod-like liquid crystal molecules are placed between the orientation layers, the alignment of the liquid crystal is easily controlled by the orientation layers.
An optical compensatory sheet (phase retarder) is often provided between the cell and the polarizing element to enlarge a viewing angle of the display. The optical compensatory sheet has another function of removing undesirable color from a displayed image. A stretched birefringent film has been usually used as the optical compensatory sheet. An optically anisotropic element has recently been proposed as an optical compensatory sheet in place of the stretched birefringent film. The optically anisotropic element comprises a transparent support and an optically anisotropic layer.
The optically anisotropic layer comprises aligned liquid crystal molecules that are aligned and polymerized while keeping alignment. The liquid crystal molecules are aligned by one orientation layer provided between the transparent support and the optically anisotropic layer.
It is difficult to align liquid crystal molecules uniformly (i.e., to orient the molecules in monodomain alignment) from an interface facing the orientation layer to the opposite interface facing air by using only one orientation layer. If the liquid crystal molecules are not uniformly aligned, disclination arises to scatter light.
An object of the present invention is to provide a liquid crystal composition in which liquid crystal molecules can easily be aligned uniformly.
Another object of the invention is to provide an optically anisotropic element in which liquid crystal molecules are uniformly aligned near an interface having no orientation layer.
A further object of the present invention is to provide a fluorine compound which can be used as an alignment promoter effectively controlling the alignment of liquid crystal molecules.
The present invention provides a liquid crystal composition comprising liquid crystal molecules and an alignment promoter represented by the formula (I) in an amount of 0.01 to 20 wt. % based on the amount of the liquid crystal molecules:
xe2x80x83(Hbxe2x80x94)mL(xe2x80x94Bu)nxe2x80x83xe2x80x83(I)
in which Hb is a hydrophobic group selected from the group consisting of a fluorine-substituted alkyl group having 1 to 40 carbon atoms, a fluorine-substituted aryl group having 6 to 40 carbon atoms, an alkyl group having 6 to 60 carbon atoms and an alkyl-substituted oligosiloxanoxy group having 1 to 60 carbon atoms; Bu is a bulky group showing an excluded volume effect and comprising at least two rings; L is an (m+n)-valent linking group; and each of m and n is independently an integer of 1 to 12.
The invention also provides an optically anisotropic element which comprises a liquid crystal layer comprising liquid crystal molecules and an orientation layer provided on only one side of the liquid crystal layer, wherein the liquid crystal layer further contains an alignment promoter represented by the formula (I) in an amount of 0.005 to 0.5 g/m2.
The invention further provides a fluorine compound represented by the formula (Ia):
Hbxe2x80x94Lxe2x80x94Buxe2x80x83xe2x80x83(Ia)
in which Hb is a fluorine-substituted alkyl group having 1 to 40 carbon atoms or a fluorine-substituted aryl group having 6 to 40 carbon atoms; Bu is a bulky group showing an excluded volume effect and comprising a tricyclic condensed ring, a tetracyclic condensed ring or at least two rings combined by a single bond, a vinylene bond or an ethynylene bond; L is a divalent linking group selected from the group consisting of an alkylene group, xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94NRxe2x80x94, xe2x80x94SO2xe2x80x94 and a combination thereof; and R is hydrogen or an alkyl group.
In the present specification, the term xe2x80x9caverage inclined anglexe2x80x9d means an average of angles between discotic planes of discotic liquid crystal molecules and a surface of the support (or a surface of the orientation layer), or an average of angles between major axes of rod-like liquid crystal molecules and a surface of the support (or a surface of the orientation layer).
The present specification refers to alignment of liquid crystal molecules at an average inclined angle in the range of 50xc2x0 to 90xc2x0 as essentially vertical alignment. The specification also refers to alignment of liquid crystal molecules at an average inclined angle in the range of 0xc2x0 to 50xc2x0 as horizontal alignment.
In other words, the term xe2x80x9cessentially vertical alignmentxe2x80x9d means homogeneous alignment of discotic liquid crystal molecules or homeotropic alignment of rod-like liquid crystal molecules. In contrast, the term xe2x80x9cessentially horizontal alignmentxe2x80x9d in the specification means homeotropic alignment of discotic liquid crystal molecules or homogeneous alignment of rod-like liquid crystal molecules.
If liquid crystal molecules are placed between a pair of orientation layers (for example, in a liquid crystal cell), the layer of the liquid crystal molecules has no free interface (an interface facing air). In that case, the alignment of the liquid crystal molecules can be controlled relatively with ease. On the other hand, however, if only one orientation layer is used, various alignment defects often appear near the free interface (having no orientation layer). Since force for controlling the alignment is not given near the free interface, it is very difficult to align the liquid crystal molecules uniformly without defects. Further, the alignment near the free interface is often perturbed by influence from outside. For example, solvent of the coating liquid unevenly evaporates from the free interface, or the free interface is blown by air for drying. Consequently, defects of the alignment are liable to appear on the free interface. Dual domain alignment is often observed on the free interface when discotic liquid crystal molecules are aligned vertically (homogeneously). The dual domain alignment comprises two aligning forms in which the discotic planes in one aligning form reversely face to those in the other aligning form though the optical axes are parallel.
The study of the present applicants revealed that the compound defined by the formula (I) has a function of controlling the alignment of liquid crystal molecules, particularly liquid crystal molecules near the free interface when only one orientation layer is used. As shown in the formula (I), the compound comprises a hydrophobic group (Hb) and a bulky group (Bu) showing an excluded volume effect.
The compound of the formula (I) is mixed with liquid crystal, applied on the orientation layer, and thickly distributed near the free interface. For thickly distributing near the free interface, the compound must be incompatible with the liquid crystal so that the phase of the compound may be separated from that of the liquid crystal. The hydrophobic group (Hb) has a function of separating the compound from the liquid crystal phase.
Further, for promoting the alignment of the liquid crystal molecules, the compound must comprise a relatively rigid part having molecular polarization property similar to that of the liquid crystal. The bulky group (Bu) functions as that part.
The applicant considers that the hydrophobic group (Hb) faces the air side while the bulky group (Bu) faces the liquid crystal side, where the compound of the formula (I) is placed near the free interface. If the bulky group (Bu) has a planar structure (for example, comprising a tricyclic or tetracyclic condensed ring), the compound effectively aligns rod-like liquid crystal molecules horizontally. Further, the compound having the bulky group (Bu) of a planar structure also aligns discotic liquid crystal molecules horizontally or vertically according to whether the group (Bu) is hydrophilic or hydrophobic, respectively. If the bulky group (Bu) has a structure comprising a part protruding to the liquid crystal side (for example, a structure comprising at least two rings connected through a single bond, a vinylene bond or an ethynylene bond), the compound vertically aligns both rod-like and discotic liquid crystal molecules.
As is described above, interactions (such as intermolecular electrostatic force of attraction and force of repulsion caused by the excluded volume effect of the bulky group (Bu)) between the liquid crystal molecules and the compound of the formula (I) can be freely controlled by varying the molecular structure, particularly the bulky group (Bu) of the compound. Regardless of the chemical structures of the liquid crystal molecules, the inclined angles of the molecules near the free interface can be freely controlled by appropriately selecting the compound of the formula (I).
Therefore, the liquid crystal molecules even near the free interface having no orientation layer can be uniformly aligned by the alignment promoter represented by the formula (I).
The compound represented by the formula (I) is used as the alignment promoter.
(Hbxe2x80x94)mL(xe2x80x94Bu)nxe2x80x83xe2x80x83(I)
In the formula (I), Hb is a hydrophobic group selected from the group consisting of a fluorine-substituted alkyl group having 1 to 40 carbon atoms, a fluorine-substituted aryl group having 6 to 40 carbon atoms, an alkyl group having 6 to 60 carbon atoms and an alkyl-substituted oligosiloxanoxy group having 1 to 60 carbon atoms. A fluorine-substituted alkyl group having 1 to 40 carbon atoms and a fluorine-substituted aryl group having 6 to 40 carbon atoms are preferred, and a fluorine-substituted alkyl group having 1 to 40 carbon atoms is particularly preferred.
The fluorine-substituted alkyl group may have a cyclic or branched structure. The fluorine-substituted alkyl group has 1 to 40 carbon atoms, preferably 2 to 30 carbon atoms, more preferably 3 to 20 carbon atoms, further preferably 4 to 15 carbon atoms, and most preferably 6 to 12 carbon atoms. The ratio of fluorine atoms substituting hydrogen atoms of the alkyl group is preferably in the range of 50 to 100%, more preferably 60 to 100%, further preferably 70 to 100%, furthermore preferably 80 to 100%, and most preferably 85 to 100%.
The fluorine-substituted aryl group has 6 to 40 carbon atoms. The fluorine-substituted aryl group is preferably a fluorine-substituted phenyl. The ratio of fluorine atoms substituting hydrogen atoms of the aryl group is preferably in the range of 50 to 100%, more preferably 60 to 100%, further preferably 70 to 100%, furthermore preferably 80 to 100%, and most preferably 85 to 100%.
The alkyl group having 6 to 60 carbon atoms may have a cyclic or branched structure. The alkyl group has preferably 7 to 50 carbon atoms, more preferably 8 to 40 carbon atoms, further preferably 9 to 30 carbon atoms and most preferably 10 to 20 carbon atoms.
The alkyl-substituted oligosiloxanoxy group comprises 1 to 60 carbon atoms in total. The alkyl-substituted oligosiloxanoxy group is represented by the following formula:
R1xe2x80x94(SiR22xe2x80x94O)qxe2x80x94
in which R1 is hydrogen, hydroxyl or an alkyl group; R2 is hydrogen or an alkyl group, and at least one of the two groups represented by R2 is an alkyl group; and q is an integer of 2 to 12. R1 preferably is hydroxyl. Each of the two groups represented by R2 preferably is an alkyl group. In the formula, q preferably is an integer of 2 to 8, and more preferably is an integer of 3 to 6.
The alkyl group may have a cyclic or branched structure. The alkyl group preferably has 1 to 12 carbon atoms, more preferably has 1 to 8 carbon atoms, further preferably has 1 to 6 carbon atoms, furthermore preferably has 1 to 4 carbon atoms, and most preferably has 1 or 2 carbon atoms.
Examples of the hydrophobic group (Hb) are shown below.
(Hb-1) n-C8F17xe2x80x94
(Hb-2) Hxe2x80x94C8F16xe2x80x94
(Hb-3) tetrafluorophenylxe2x80x94
(Hb-4) Hxe2x80x94C6F12xe2x80x94
(Hb-5) Hxe2x80x94C4F8xe2x80x94
(Hb-6) HOxe2x80x94(Si(CH3)2xe2x80x94O)4xe2x80x94
(Hb-7) n-C12H25xe2x80x94
In the formula (I), Bu is a bulky group showing an excluded volume effect and comprising at least two rings. Each ring may be an aliphatic, aromatic or hetero-cyclic ring. Further, the ring is preferably a five-, six- or seven-membered, more preferably five- or six-membered, and further preferably six-membered ring. The rings may form a condensed ring, or be connected through a spiro bond, a single bond or a divalent linking group. Preferably, they form a condensed ring, or are connected through a single bond or a divalent linking group.
In the case that at least two of the rings form a condensed ring, they preferably form a tricyclic or tetracyclic condensed ring.
In the case that at least two of the rings are connected through a divalent linking group, examples of the linking group include xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, an alkylene group, vinylene bond (xe2x80x94CHxe2x95x90CHxe2x80x94), ethynylene bond (xe2x80x94Cxe2x89xa1Cxe2x80x94) and a combination thereof. The linking group is preferably vinylene bond or ethynylene bond, and more preferably ethynylene bond.
The ring can have a substituent group. Examples of the substituent group include halogen atom, hydroxyl, cyano, nitro, an alkyl group (preferably comprising 1 to 5 carbon atoms), a substituted alkyl group (e.g., carboxyalkyl group, alkoxyalkyl group), an alkoxy group, a substituted alkoxy group (e.g., oligoalkoxy group), an alkenyloxy group (e.g., vinyloxy), an acyl group (e.g., acryloyl, methacryloyl), an acyloxy group (e.g., acryloyloxy, benzoyloxy) and an epoxy group (e.g., epoxyethyl).
Examples of the bulky group (Bu) showing an excluded volume effect are shown below. 
In the formula (I), L is an (m+n)-valent linking group.
In the formula (I), each of m and n is independently an integer of 1 to 12, preferably 1 to 8, more preferably 1 to 6, further preferably 1 to 4, furthermore preferably 1 to 3, and most preferably 1.
In the case that each of m and n is 1, L is preferably a divalent linking group selected from the group consisting of an alkylene group, xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94NRxe2x80x94, xe2x80x94SO2xe2x80x94, and a combination thereof. Here R is a hydrogen or an alkyl group. Preferably, L contains at least one polar group (group other than -alkylene group-).
The above alkylene group preferably comprises 1 to 40, more preferably 1 to 30, further preferably 1 to 20, furthermore preferably 1 to 15 and most preferably 1 to 12 carbon atoms.
The above alkyl group preferably comprises 1 to 40, more preferably 1 to 30, further preferably 1 to 20, furthermore preferably 1 to 15 and most preferably 1 to 12 carbon atoms.
In the case that m is 2 or more, plural hydrophobic groups (Hb) may be different from each other.
In the case that n is 2 or more, plural bulky groups (Bu) may be different from each other.
If m or n is not less than 2, the linking group (L) may have a chain structure or a cyclic structure. In the case that the linking group (L) has a chain structure, the plural hydrophobic groups (Hb) or bulky groups (Bu) may connect to the main chain of the linking group (L) as side chains. In the case that the linking group (L) has a cyclic structure, the plural hydrophobic groups (Hb) or bulky groups (Bu) may connect to the ring of the linking group (L) as substituent groups.
If each of m and n is independently an integer of 2 to 12, the linking group (L) preferably has a cyclic structure.
Examples of the linking group (L) are shown below. In each example of divalent linking groups (L-1 to L-11), the left side is adjacent to the hydrophobic group (Hb), and the right side is adjacent to the bulky group (Bu). In each example of polyvalent linking groups (L-12 to L-17), the hydrophobic group (Hb) and the bulky group (Bu) are shown.
(L-1) xe2x80x94SO2xe2x80x94N(n-C3H7)xe2x80x94C2H4xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94
(L-2) xe2x80x94SO2xe2x80x94N(n-C3H7)xe2x80x94C2H4xe2x80x94Oxe2x80x94COxe2x80x94NHxe2x80x94
(L-3) xe2x80x94SO2xe2x80x94N(n-C3H7)xe2x80x94C2H4xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94CH2xe2x80x94
(L-4) xe2x80x94SO2xe2x80x94NHxe2x80x94C2H4xe2x80x94Oxe2x80x94COxe2x80x94NHxe2x80x94
(L-5) xe2x80x94SO2xe2x80x94NHxe2x80x94C2H4xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94
(L-6) xe2x80x94COxe2x80x94NHxe2x80x94C2H4xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94
(L-7) xe2x80x94COxe2x80x94NHxe2x80x94
(L-8) xe2x80x94NHxe2x80x94C2H4xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94
(L-9) xe2x80x94COxe2x80x94NHxe2x80x94C2H4xe2x80x94Oxe2x80x94COxe2x80x94NHxe2x80x94
(L-10) xe2x80x94SO2xe2x80x94N(n-C3H7)xe2x80x94C6H12xe2x80x94Oxe2x80x94COxe2x80x94NHxe2x80x94
(L-11) xe2x80x94C2H4xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94
The alignment promoter is preferably represented by the formula (Ia):
Hbxe2x80x94Lxe2x80x94Buxe2x80x83xe2x80x83(Ia)
In the formula (Ia), Hb is a fluorine-substituted alkyl group having 1 to 40 carbon atoms or a fluorine-substituted aryl group having 6 to 40 carbon atoms.
In the formula (Ia), Bu is a bulky group showing an excluded volume effect and comprising a tricyclic condensed ring, a tetracyclic condensed ring or at least two rings combined by a single bond, a vinylene bond or an ethynylene bond.
In the formula (Ia), L is a divalent linking group selected from the group consisting of an alkylene group, xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94NRxe2x80x94, xe2x80x94SO2xe2x80x94 and a combination thereof. R is hydrogen or an alkyl group.
The alignment promoter is a compound consisting of the aforementioned hydrophobic group (Hb), the bulky group (Bu) showing an excluded volume effect and the linking group (L). There is no specific limitation on the combination thereof.